Polarization mode dispersion (PMD) is an important consideration when dealing with an optical fiber used in a fiber-optic communication system and it can be generated by concentricity error or the non-circularity of the core of an optical fiber, or by external stress or the like. Polarization mode dispersion is a group delay difference between two orthogonal polarization modes of propagated light along an optical fiber. The larger the polarization mode dispersion of an optical fiber, the more the pulse width is spread by this group delay difference. Therefore, polarization mode dispersion is one factor preventing high-speed transmission. As a result, when planning and constructing a fiber-optic communication system, the planner or constructor should comprehend or measure values of the PMD of the optical fibers used in the system. An interferometric method and a Jones Matrix Eigenanalysis method and the like are well-known measuring methods for the PMD of an optical fiber.
However, using the above-mentioned conventional PMD measuring methods, it is only possible to measure cumulative PMD over the whole length of the measured optical fiber. Consequently, using these methods, it is impossible to detect where individual sections of an optical fiber may have sizable PMD. Therefore, for example, if one wants to increase the transmission speed of a transmission line (i.e. an optical fiber) including that of a section or sections having sizable PMD, either the entire line or individual sections of the line should be replaced. However, using the above-described methods, in order to replace only those sections having high PMD, it is necessary to break the line into short sections and measure the PMD of each section separately. Neither of these methods have proved to be a good solution for reducing PMD and increasing speed in the line.
Moreover, in the case of manufacturing an optical cable from multiple optical fibers, it is not possible to sort out those sections having sizable PMD in advance. As a result, in order to reduce PMD, it has been necessary to measure the polarization mode dispersion of all the fibers in each cable (which may include sections with sizable PMD) after the formation of the cable in order to make an inspection thereof. The inspection and measurement of the PMD of such a cable unnecessarily increases the cost of manufacturing the cable.
Furthermore, when measuring PMD by one of the above mentioned methods, it is necessary to create a light incident section at one end of the optical fiber and to create a light detect section at the other end thereof. Therefore, when measuring the PMD of a predetermined section of an existing line or the like, there is often a long distance between the light incident section and the light detect section, and synchronization of the measuring system becomes difficult.
To solve these problems, methods of measuring the longitudinal distribution of PMD using Rayleigh scattered light have been proposed in, for example, “Journal of Lightwave Technology”, B. Hattner et. al., Volume 17, No. 10, pp. 1843–1848, “Proceedings of the 2002 IEICE General Conference”, 2002, Tomioka et. al., B-10-113, and Japanese Unexamined Patent Application, First Publication No. 2000-329651.
However, because these methods require high-cost devices such as a tunable light source, a polarization controller, a polarization analyzer, a phase detector or the like, the costs of the apparatuses are unreasonably high for actual use. Moreover, in order to perform measurements for a plurality of polarization states long measurement times or a plurality of detectors are required, either of which also results in unnecessarily high costs.